// Time Complexity: O(n)
// Space Complexity: O(1)

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
    public:
        bool hasCycle(ListNode *head) {
            ListNode *slow = head, *fast = head;

            while(fast && fast->next) {
                slow = slow->next;
                fast = fast->next->next;

                if(slow == fast)
                    return true;
            }

            return false;
        }
};
